The present invention refers to an apparatus for measuring the pressure of a fluid, and in particular to a pressure measuring apparatus of a type having a pressure sensor which contacts a first pressure transmission element, and a second pressure transmission element which is in communication with the fluid of which the pressure is to be measured.
Such a pressure measuring apparatus is utilized for all kinds of fluids, e.g. in the medical field for pressure determination in extracorporeal blood circulations as encountered during dialysis.
In an apparatus for measuring the pressure of a fluid in a circulation, e.g. of blood in extracorporeal blood circulations, the provision of a branch from the actual circulation of the fluid for measurement is disadvantageous because the simple and clear line configuration of the circulation is lost and the complexity increases the chances of error and the assembly time of the system. Moreover, the arrangement of a branch requires a greater filling volume of the system. Safety concerns necessitate however to reduce the quantity of extracorporeal blood volume to a minimum. Also, a pressure measuring process which operates in a branch of the blood circulation draws a certain blood volume which is retained in a dead space for a relative long period. This extended retention time increases the tendency of blood coagulation and an increased administration of systemic coagulation inhibitors, such as heparin, becomes necessary.
Non-invasive systems do not experience these drawbacks. By measuring the pressure not in a branch but directly upon the tube, the line configuration of the tube system remains clear, the filling volume is only insignificantly increased and the dead space of the branch is omitted.
Tubes, such as blood tubings, have elastic as well as viscous or elasticoviscous properties. While the initial state is recovered after an elastic stress, this is not the case during a viscous modification. An absolute pressure measurement upon the tube is complicated in particular through these viscous properties of the tube material. In order to minimize the impact of the viscosity of the material, the use of a thin membrane is proposed instead of a thick tubing in the area of the pressure measurement. However, problems were experienced in connection with the transmission of the membrane movement upon the pressure sensor. Moreover, a further problem arose also because generally, also very high negative pressures should be detected.
European Pat. No. EP 0 355 373 B1 discloses a fluid-pressure detector, in which a measuring unit detects the expansion or shrinkage of a pillow-like body capable of expanding and shrinking depending on the pressure of the fluid flowing in the pillow-like body. The measurement on the arched surface of the pillow-like body is effected by means of an iron plate, which is attached to the pillow-like body, via a magnet which is detachably secured to the tip of the measuring unit. This detector is complicated because the iron plate is connected with the part conducting the fluid and thus becomes too cost-intensive in one-way systems, such as extracorporeal blood circulations, after a one-time use. Moreover, the inherent inertia of such a system results in a relatively poor sensitiveness of the system.
European Pat. No. 0 330 891 B1 discloses an arrangement for pressure transmission in which an outer casing is divided by a membrane into two separate spaces, with the fluid whose pressure is to be determined flowing through one space, and with a second, auxiliary fluid flowing through the other space. Pressure fluctuations of the fluid are transmitted by the membrane to the second fluid for effecting the actual pressure measurement in known invasive manner. Because of the indirect pressure measurement and because of the inherent inertia, also this system is not very sensitive and reacts in a relatively slow manner.
International patent application WO 93/22641 A1 describes a pressure measuring system in which pressure changes are transmitted from a membrane onto a pressure sensor by a fluid. This system has the same drawbacks as described above, and moreover is relatively complicated and cumbersome to manufacture so that its use is impractical for one-way systems.
European Pat. No. EP 0 200 709 B1 describes a pressure transducer provided with a plate membrane which is permanently attached to a housing and seals the latter against the fluid being measured. The membrane is connected to a measuring element situated in the housing for transmitting the pressure to be determined. An annular area of the membrane is of arched or conical shape in its original state and extends into the interior of the housing, when it is not subject to the pressure being measured. The configuration of the membrane complicates the manufacture and deteriorates the contact between the sensor membrane and the tube membrane.
European Pat. No. 0 130 441 B1 discloses a pressure measuring system which operates without any transfer fluid. The cavity between both pressure transmission elements is emptied or evacuated by a vacuum pump in order to effect a contact of the pressure transmission elements. This system is highly susceptible to failure, very complicated and thus expensive. Moreover, the system is coupled via a tube so that the fluid is conducted through the measuring unit and thus has all those drawbacks of an invasive system as previously described.
German Pat. No. DE-C-29 30 869 discloses a pressure measuring cell by which two membranes are contacted through twisting of the housing parts. Since the pressure measuring cell and the pressure transducer are manually bolted together, the attainable pressure forces between both membranes are relatively low. Moreover, the twisting motion for attachment of both membranes results in a membrane torsion which adversely affects the desired tightness and may even lead to a destruction of the membrane. This however must be prevented under any circumstances when used in the medical field.